This invention relates to the alloy art and has particular relationship to cobalt-base alloys particularly suitable for use in apparatus operating at high temperature typically at 1500.degree. F to 1900.degree. F. Typical of such apparatus are the parts of gas-turbines such as the stationary blades and the vanes of large cross section typically of about 1 inch maximum thickness. Such blades and vanes are produced by investment casting. The alloy is molten in a crucible and poured into a mold. The molded structure is coated with an oxidation-sulfidation resistant coating. Typical of the prior art are the alloys disclosed in Wheaton U.S. Pat. No. 3,432,294 and discussed in the documents listed above. In the use of the Wheaton and like alloys the difficulty has been experienced that the surface carbide is oxidized. The surface of the molded article then has oxidized areas and the oxidation-sulfidation resistant coating cannot be applied effectively to such areas. In addition the affinity to oxidation of the surface carbide causes the alloy to attack and act with the crucible in which it is molten and the mold excessively and the result is inclusive in the castings request renewal of the crucible and mold at substantial cost is required.
The parts operating at high temperatures which are composed of the Wheaton alloy require high creep-rupture strength and to achieve this high creep-rupture strength the Wheaton alloy includes, among the elements of which it is composed, zirconium and titanium. Typically, there is 0.1% to 1% zirconium and 0.1% to 0.5% titanium. Attempts have been made to reduce the surface-carbide oxidation by reducing the zirconium in the alloy but this has failed to entirely eliminate the oxidation and its attendant difficulties.
It is an object of this invention to overcome the above-described difficulties of the prior art and to provide a cobalt-base alloy for use in casting parts of apparatus that operate at high temperatures which alloy shall have high creep resistance at the high temperatures and in the fusing and molding of which detrimental surface-carbide oxidation shall not occur.